From: "Kevin Patterson"
Subject: Re: 555 Monostable?
Date: Sat, 9 Nov 2002 10:24:58 -0000
NNTP-Posting-Date: Sat, 9 Nov 2002 10:26:18 +0000 (UTC)
X-Newsreader: Microsoft Outlook Express 6.00.2600.0000
The diode method worked for me, my way was to use a relay, controlled by the
trigger transistor to earth the charging capacitor :-), but this works fine
I've noticed that the capacitor is not fully discharged by the diode (down
to 0.55v), this is properly down to the diode push voltage.
"Tom Bruhns" wrote in message
> "Kevin Patterson" wrote in message
> > The 555 circuit that I have made is a simple monostable timer, that
> > a relay on the output(pin3). My problem is that the timing interval
> > on the first press of the trigger (pin2), any other trigger presses
> > the time interval don't restart the timer.
> > My scienrio is this, I want to enable a relay for a set period (4
> > if the trigger button is pressed within this 4 seconds I would like the
> > timer to start again (i.e. 4 seconds after the last press)
> > The circuit can be found at this link
> > Does anyone know how I could modify my monostable 555 circuit (NE555N
> > to allow retriggering from the trigger input.
> If you simply put a diode between pins 2 (cathode) and 6 (anode) of
> that circuit, it should do what you want. You might also put a small
> resistance in series with the diode to limit the current. Diode
> leakage current should be small compared with the current in R, so if
> R is 1 megohm, be sure the diode leakage current is under a microamp
> or so. The series resistance to limit the current would be chosen
> from Rlimit = Vs*2/(3*diode_peak_current_rating). For example, if you
> can use a 1N4004 diode and it has a peak current rating of 30A and
> Vs=15V, you should use 1/3 ohm resistance minimum. 10 ohms would be
> fine in this case. (You should probably pick the lower of the diode
> peak current rating and a safe current rating for the switch that
> won't decrease its life through pitting of the contacts.) If R is
> 100kohms and C is 47uF in your circuit, then the 10 ohm resistor will
> discharge C in a millisecond or so, so you don't have to worry about
> holding the switch down for a long time...a very short tap would do.